<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- This file documents the use of the GNU compilers.

Copyright (C) 1988-2023 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Funding Free Software", the Front-Cover
Texts being (a) (see below), and with the Back-Cover Texts being (b)
(see below).  A copy of the license is included in the section entitled
"GNU Free Documentation License".

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU
     software.  Copies published by the Free Software Foundation raise
     funds for GNU development. -->
<!-- Created by GNU Texinfo 6.5, http://www.gnu.org/software/texinfo/ -->
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Cast to Union (Using the GNU Compiler Collection (GCC))</title>

<meta name="description" content="Cast to Union (Using the GNU Compiler Collection (GCC))">
<meta name="keywords" content="Cast to Union (Using the GNU Compiler Collection (GCC))">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<link href="index.html#Top" rel="start" title="Top">
<link href="Indices.html#Indices" rel="index" title="Indices">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">
<link href="Mixed-Labels-and-Declarations.html#Mixed-Labels-and-Declarations" rel="next" title="Mixed Labels and Declarations">
<link href="Case-Ranges.html#Case-Ranges" rel="prev" title="Case Ranges">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.indentedblock {margin-right: 0em}
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smalllisp {margin-left: 3.2em}
kbd {font-style: oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: inherit; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: inherit; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.nolinebreak {white-space: nowrap}
span.roman {font-family: initial; font-weight: normal}
span.sansserif {font-family: sans-serif; font-weight: normal}
ul.no-bullet {list-style: none}
-->
</style>


</head>

<body lang="en_US">
<a name="Cast-to-Union"></a>
<div class="header">
<p>
Next: <a href="Mixed-Labels-and-Declarations.html#Mixed-Labels-and-Declarations" accesskey="n" rel="next">Mixed Labels and Declarations</a>, Previous: <a href="Case-Ranges.html#Case-Ranges" accesskey="p" rel="prev">Case Ranges</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Cast-to-a-Union-Type"></a>
<h3 class="section">6.31 Cast to a Union Type</h3>
<a name="index-cast-to-a-union"></a>
<a name="index-union_002c-casting-to-a"></a>

<p>A cast to a union type is a C extension not available in C++.  It looks
just like ordinary casts with the constraint that the type specified is
a union type.  You can specify the type either with the <code>union</code>
keyword or with a <code>typedef</code> name that refers to a union.  The result
of a cast to a union is a temporary rvalue of the union type with a member
whose type matches that of the operand initialized to the value of
the operand.  The effect of a cast to a union is similar to a compound
literal except that it yields an rvalue like standard casts do.
See <a href="Compound-Literals.html#Compound-Literals">Compound Literals</a>.
</p>
<p>Expressions that may be cast to the union type are those whose type matches
at least one of the members of the union.  Thus, given the following union
and variables:
</p>
<div class="smallexample">
<pre class="smallexample">union foo { int i; double d; };
int x;
double y;
union foo z;
</pre></div>

<p>both <code>x</code> and <code>y</code> can be cast to type <code>union foo</code> and
the following assignments
</p><div class="smallexample">
<pre class="smallexample">  z = (union foo) x;
  z = (union foo) y;
</pre></div>
<p>are shorthand equivalents of these
</p><div class="smallexample">
<pre class="smallexample">  z = (union foo) { .i = x };
  z = (union foo) { .d = y };
</pre></div>

<p>However, <code>(union foo) FLT_MAX;</code> is not a valid cast because the union
has no member of type <code>float</code>.
</p>
<p>Using the cast as the right-hand side of an assignment to a variable of
union type is equivalent to storing in a member of the union with
the same type
</p>
<div class="smallexample">
<pre class="smallexample">union foo u;
/* <span class="roman">&hellip;</span> */
u = (union foo) x  &equiv;  u.i = x
u = (union foo) y  &equiv;  u.d = y
</pre></div>

<p>You can also use the union cast as a function argument:
</p>
<div class="smallexample">
<pre class="smallexample">void hack (union foo);
/* <span class="roman">&hellip;</span> */
hack ((union foo) x);
</pre></div>

<hr>
<div class="header">
<p>
Next: <a href="Mixed-Labels-and-Declarations.html#Mixed-Labels-and-Declarations" accesskey="n" rel="next">Mixed Labels and Declarations</a>, Previous: <a href="Case-Ranges.html#Case-Ranges" accesskey="p" rel="prev">Case Ranges</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p>
</div>



</body>
</html>
